The present invention relates to subterranean treatment fluids. In particular, the present invention relates to variable density subterranean treatment fluid compositions and methods for using such compositions.
A treatment fluid may be used in a subterranean formation in a variety of ways. For example, a fluid may be used to drill a borehole in a subterranean formation, to stimulate a well bore in a subterranean formation, and to clean up a well bore in a subterranean formation, as well as for numerous other purposes. The process of drilling a well typically requires the use of a treatment fluid referred to as a “drilling fluid.” During the drilling process, the drilling fluid passes down through the inside of the drill string, exits through the drill bit, and returns to the drill rig through the annulus between the drill string and well bore. The circulating drilling fluid, inter alia, lubricates the drill bit, carries drill cuttings to the surface, and balances the formation pressure exerted on the well bore. Additionally, a properly prepared drilling fluid enhances well drilling safety by preventing “kicks.” A kick is an uncontrolled flow of formation fluid into the well bore from the subterranean formation typically resulting from drilling into a zone of higher-than-expected or unanticipated pressure. Kicks may be both dangerous and very costly to drillers. Thus, drilling operators inherently wish to avoid or minimize kicks, or at least detect kicks as early as possible.
Drilling offshore in deep waters requires specially blended drilling fluids which must be carefully formulated to allow for, inter alia, the narrow range of pressure separating the pore pressure from the fracture gradient of the formation. As used herein, the term “pore pressure” refers to the pressure exerted on the borehole by fluids within the formation, while the term “fracture gradient” refers to that pressure that will fracture the formation. Such fracturing of the formation could result in flow of drilling fluids out of the borehole into the formation (commonly referred to as “lost circulation”), or possibly an uncontrolled blowout of the formation fluids, and in some cases, fluids and particulates. Accordingly, the weight of the drilling fluid preferably should be sufficient to balance the pore pressure (and thus provide the necessary protection against kicks) without inadvertently fracturing the sediment and rock around the drill bit.
The formulation of a drilling fluid satisfying these limitations is often complicated because the riser connecting the borehole to the drilling rig must often be quite long, particularly when drilling in deep water applications. As used herein, the term “riser” refers to a length of piping connecting the borehole to the drill ship or drilling rig, through which the drilling fluid is returned to the surface. The length of the riser column could be long, and thus the pressure exerted due to friction and hydrostatic forces could be considerable, even when at relatively shallow depths within the subterranean formation and even when using low density drilling fluids. Further complicating the formulation of the drilling fluid, at subsequent depths the hydrostatic and friction forces may be insufficient to prevent fluid influx from the formation.
Efforts to overcome this difficulty in drilling deep water wells have met with limited success. One proposed solution involves the installation of multiple strings of casing within the borehole to guard against inadvertent fracturing of the formation during the drilling process. This strategy is problematic because the installation of additional casing usually correspondingly reduces the usable diameter of the borehole. Furthermore, the installation of additional strings of casing may add greatly to the expense and complication of completing the well. Another option requires placement of pumps on the ocean floor, wherein drilling fluid exiting the well bore is diverted from the drilling riser annulus into the pumps. Subsequently, the pumps return the drilling fluid to the drill ship through additional pipes. Accordingly, in this method, the pumps remove the weight of the returning drilling fluid from the well bore. However, this method may not achieve desirable results because of the great expense involved in procuring and installing the system of risers and pumps. Another proposed option has been to reduce the density of the drilling fluid as it exits the well bore by injecting hollow rigid spheres into the drilling fluid to reduce the density of the drilling fluid. However, this method is problematic, inter alia, because of the cost and mechanical difficulty of injecting these spheres at the sea floor, and then separating them from the drilling fluid at the surface. Furthermore, the incompressible nature of these rigid spheres tends to cause them to crush at the pressures typically encountered in deepwater drilling; accordingly, such rigid spheres are typically unable to reduce the density of the drilling fluid while it travels upward toward the surface.
Analogous problems exist in other subterranean operations. For example, a fracturing or acidizing operation may involve additional expense in removing the spent fracturing or acidizing fluid from the well bore after the operation is conducted, due in part to the fixed-density nature of such fluids, which may necessitate additional manpower or pumping equipment to remove such fluid in a timely fashion.